The term "image recording" is conventionally taken to mean a process which produces a spatial pattern of optical absorption in the recording medium. Photographic processes are well known examples of this type of process.
In a broader sense, however, the word "image" means a spatial variation of the optical properties of a sample in such a way as to cause a desired modification of a beam of light passing through the sample. Refractive index images in general, and holograms in particular, which modulate the phase, rather than the amplitude, of the beam passing through them, are usually referred to as phase holograms. Phase holographic image recording systems produce a spatial pattern of varying refractive index rather than optical absorption in the recording medium and, thus, can modulate a beam of light without absorbing it.
This type of refractive index image also includes a number of optical elements or devices which superficially bear little resemblance to absorption images. Examples are holographic lenses, gratings, mirrors, and optical waveguides.
Holography is a form of optical information storage. The general principles are described in a number of references, e.g., "Photography by Laser" by E. N. Leith and J. Upatnieks in SCIENTIFIC AMERICAN, 212, No. 6,24-35 (June, 1965). A useful discussion of holography is presented in "Holography", by C. C. Guest, in Encyclopedia of Physical Science and Technology, Vol. 6, pp. 507-519, R. A. Meyers, Ed., Academic Press, Orlando, Fla., 1987. In brief, the object to be photographed or imaged is illuminated with coherent light, e.g., from a laser, and a light sensitive recording medium, such as a photographic plate, is positioned so as to receive light reflected from the object. Each point on the object reflects light to the entire recording medium, and each point on the recording medium receives light from the entire object. This beam of reflected light is known as the object beam. At the same time, a portion of the coherent light is beamed by a mirror directly to the recording medium, bypassing the object. This beam is known as the reference beam. What is recorded on the recording medium is the interference pattern that results from the interaction of the reference beam and the object beam impinging on the medium. When the processed recording medium is subsequently illuminated and observed appropriately, the light from the illuminating source is diffracted by the hologram to reproduce the wave-front that originally reached the recording medium from the object, so that the hologram resembles a window through which the virtual image of the object is observed in full three-dimensional form, complete with parallax.
Holograms that are formed by allowing the reference and object beams to enter the recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are normal or near normal to the plane of the recording medium. When the hologram is played back by viewing with transmitted light, these fringes refract the light to produce the viewed virtual image. Such transmission holograms may be produced by methods which are well known in the art such as disclosed in U.S. Pat. No. 3,506,327; U.S. Pat. No. 3,838,903 and U.S. Pat. No. 3,894,787.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides, so that they are traveling in approximately opposite directions are known as reflection holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, planes parallel to the plane of the recording medium. When the hologram is played back, these fringes act as mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection rather than in transmission. Since the wavelength sensitivity of this type of hologram is very high, white light may be used for reconstruction. Reflection holograms may be produced by an on-axis method wherein the beam of coherent radiation is projected through the recording medium onto an object therebehind. In this instance, the reflected object beam returns and intersects with the projected beam in the plane of the recording medium to form fringes substantially parallel to the plane of the medium.
A diffraction grating is the simplest possible transmission hologram. It is the hologram of two coherent plane waves. It can be created by splitting a single laser beam and recombining the beams at the recording medium.
The interference pattern produced by two plane waves which are coherent and are not polarized perpendicular to each other is a set of uniformly spaced fringes with a sinusoidal intensity distribution. When incident on a recording medium they produce a set of uniformly spaced fringes which have a sinusoidal variation in refractive index, generally referred to as a grating, oriented parallel to the bisector of the angle between the two beams. If the two waves are incident at equal angles with respect to the surface of the recording medium and are both incident on the same side of the recording medium, the fringes are perpendicular to the surface of the medium and the grating is said to be unslanted. The hologram grating produced is said to be a transmission grating since light passing through it is diffracted. The grating is said to be thick if it is much thicker than the distance between the fringes, generally referred to as the grating spacing.
A diffraction grating can be characterized by its diffraction efficiency, that is the percent of incident radiation which is diffracted, and by its thickness. A simple but useful theory for thick hologram gratings, generally known as the "coupled wave theory", has been developed by Kogelnik (H. Kogelnik, Coupled wave theory for thick hologram gratings, Bell Syt. Tech. J., 48, 2909-2947, 1969). This theory treats the relationship between diffraction efficiency, grating thickness, wavelength of incident radiation, and the angle of incident radiation. A useful discussion of this theory in regard to refractive index recording systems has been presented in Section II of an article by Tomlinson and Chandross (W. J. Tomlinson and E. A. Chandross, Organic photochemical refractive-index image recording systems, Adv. in Photochem., Vol. 12, J. N. Pitts, Jr., G. S. Hammond, and K. Gollinick, eds., Wiley-Interscience, New York, 1980, pp 201-281).
Refractive index modulation is a quantitative measure of the change in refractive index between image and non-image portions of a hologram or other recording medium containing a refractive index image. For the diffraction grating, refractive index modulation is the measure of the amplitude of the sinusoidal modulation of the refractive index within the recording medium produced when the holographic image is recorded. The refractive index modulation, or index modulation, for a recording medium is best determined by holographically forming a grating in the medium and calculating the index modulation using Kogelnik's coupled wave theory and the measured parameters of the grating formed, i.e., the diffraction efficiently, medium thickness, etc.
A holographic mirror is the simplest possible reflection hologram. It can be created by splitting a single laser beam and recombining the beams at the recording medium, or the unsplit laser beam can be projected through the medium onto a plane mirror therebehind. A set of uniformly spaced fringes with a sinusoidal-like intensity distribution is formed which are oriented parallel to the bisector of the obtuse angle between the two beams propagating in the recording medium. If the obtuse angle is 180.degree. and the beams are normal to the plane of the medium, the fringes will be parallel to the plane of the medium. If the two beams do not make equal angles with the normal to the plane of the medium, then the fringes which are formed will be slanted at an acute angle relative to the plane of the medium. The holographic mirror can be characterized by its wavelength of maximum reflection and by its reflection efficiency, that is the percent of incident radiation which is reflected at its wavelength of maximum reflection.
A variety of materials have been used to record volume holograms. Among the more important are: silver halide emulsions, hardened dichromated gelatin, ferroelectric crystals, photopolymers, photochromics and photodichroics. Characteristics of these materials are given in Volume Holography and Volume Gratings, Academic Press, New York, 1981 Chapter 10, pp. 254-304 by L. Solymar and D. J. Cook.
Dichromated gelatin is the material most widely used for recording volume holograms. This material has become the popular choice because of its high diffraction efficiency and low noise characteristics. However, the material has poor shelf life and requires wet processing. Plates must be freshly prepared, or prehardened gelatin must be used. Wet processing means that an additional step is required in hologram preparation and may also cause the hologram to change due to swelling and then shrinkage of the gelatin during processing. The requirement that plates by freshly prepared each time a hologram is made, plus the problems associated with wet processing, make reproducibility extremely difficult to achieve with dichromated gelatin.
While early holograms where prepared from silver halide, liquid photopolymers, or dichromated colloids which required several processing steps, solid photopolymerizable elements were proposed which require only a single process step. U.S. Pat. 3,658,526, to Haugh discloses preparation of stable high-resolution holograms from solid photopolymerizable layers by a single step-process wherein a permanent refractive index image is obtained by a single imagewise exposure of the photopolymerizable layer to actinic radiation bearing holographic information. The holographic image formed is not destroyed by subsequent uniform actinic exposure, but rather is fixed or enhanced.
Although the solid photopolymerizable layers proposed by Haugh offer many advantages over the prior art, their efficiency is low. These layers typically have a refractive index of modulation in the range of 0.001 to 0.003. As a result, reconstructed holographic images formed in thin layers of the photopolymer only have limited brightness. While brightness can be increased by employing thicker layers of the photopolymer, this solution results in a substantial reduction to the viewing angle and causes the manufacturer to use much more of the expensive photopolymer. It also should be noted that the coated layers proposed by Haugh generally cannot be stored at room temperature for extended times without loss of speed and diffraction efficiency. Thus, there continues to be a need for improved photopolymer compositions and elements for refractive index imaging applications, including holography.